a) Field of the Invention
The present invention is directed to an arrangement for measuring the diffuse reflection of samples, particularly in process measurement technique, and to a method for the internal recalibration of the measuring head.
b) Description of the Related Art
Numerous solutions using spectrometric arrangements for process monitoring are known from the prior art. While no problems arise from measurements under laboratory conditions when standardized conditions are maintained, measurement tasks in process monitoring are subject to special requirements.
Measuring heads for the measurement of diffuse reflections require the two standards, black and white, as measuring means for calibration. The measurement scale is calibrated at the start of the measurement by means of these two standards. The samples to be measured subsequently fall between the two limiting values. Any changes in the measurement conditions can lead to erroneous results.
For example, such changes may result from deposits on the light source due to aging so that the intensity and color temperature of the light can change. Further, thermal influences on the detection system are also possible. These can lead to variations in sensitivity and in the dark signal and can accordingly falsify the measurement results. To prevent this, it is necessary to recalibrate at regular time intervals with the above-mentioned two standards.
As a rule, the measurement sample must be removed from the sample plane and replaced by the two calibrating standards. In process measurement technology this process is not only disruptive and time-consuming but, in some cases, cannot even be implemented. Another possibility consists in positioning the measuring head exactly over the calibrating sample at a location other than the sample measurement location.
U.S. Pat. No. 5,982,501 A describes a device for measuring the reflectivity of a sample whose measured values are preferably fed to a spectrometer. The measurement device comprises a housing enclosing a measuring head for measuring the light reflected by the object, an opto-electric converter for converting the measured light into electrical signals, a computer unit for processing the electrical signals and for controlling the device, a display unit for displaying the measured values, and an operating unit. When the measurement process is initiated, the measuring head is displaced by a motor from its rest position in the housing to the measurement position outside the housing, and the measurement is carried out. The measuring head then moves to the rest position in the housing again. Reference patterns for calibrating the measuring head are arranged inside the housing. For this purpose, the measuring head is positioned over the respective reference pattern and the corresponding measurement is initiated. It is possible to calibrate based on the reference values stored in the computer unit. Other color standards can be used in addition to a white standard as reference patterns for checking the spectral calibration. Further, the arrangement has a filter wheel which can be displaced together with the measuring head to spectrally influence either the light of the measurement light source or the light reflected by the object. For this purpose, the filters of the filter wheel are positioned in the respective beam path. This solution is disadvantageous in that the calibration takes place in the housing rather than directly at the measurement location. The results of the calibrations can be transferred from the interior of the housing to the external measurement location only with difficulty due to the correspondingly different conditions. Further, the entire measuring head must be exactly positioned on the calibration standard and on the measurement sample by motor in this solution.
A method and a corresponding arrangement for automatic calibration of a color detection system is described in EP 0 010 940 A1. The solution is used to monitor the color of a product and/or use it as a criterion for sorting. The principal areas of application are, for example, monitoring fried potato chips or sorting fruits or vegetables. For this purpose, the products are illuminated by light of a defined wavelength, and the light reflected by the products is evaluated. The solution describes an automatic calibration method for compensating for the influence of fluctuations in color temperature of the light source or random changes in the measurement process. To calibrate the arrangement, a disk with a known color pattern is swiveled in front of the measuring head and the intensity of the reflected light is measured. The measured light signal is compared to a known standard signal so that changes can be determined. This solution is disadvantageous in that it lacks the universality that is only ensured by the spectral information. The color patterns used for calibration are specific to the application.
Patent Application DE 10 2004 021 448.4, which has not yet been published, describes a spectrometric reflection measuring head with internal recalibration in which at least two standards, preferably a black standard and a white standard, are additionally provided in the housing of the measuring head for internal recalibration and can be swiveled selectively into the beam path of the reflection measuring head. After the measurement data of the two standards is acquired by the spectrometer, the recalibration of the reflection measuring head is carried out by the spectrometer by means of the controlling and evaluating unit. In addition, at least two external standards can be provided for calibrating the reflection measuring head before putting the measuring arrangement into operation or at determined time intervals.